Enterohemorrhagic Escherichia coli (EHEC), principally serotype O157:H7, cause an estimated 20,000 cases of diarrheal disease in the United States per year. 2-6 percent of the infected individuals, mostly young children progress to a severe renal disease, hemolytic uremic syndrome (HUS). The EHEC pathogenic factors that lead to bloody colitis and HUS are poorly understood, but knowledge of some mechanisms has recently emerged. Intimin-mediated adherence and type III effectors are encoded by a chromosomal locus termed LEE. The phage-encoded Shiga toxins (Stxs) are responsible for significant aspects of EHEC disease. EHEC strains commonly possess large plasmids, the prototype being pO157. We have identified a new pO157 gene, stcE, which encodes an extracellular zinc-metalloendoprotease (ZMP) that specifically cleaves the critical anti-inflammatory regulator C l-esterase inhibitor (C 1-Inh). C 1-Inh is a serine protease inhibitor (serpin) that provides the principal inhibition of the proteolytic cascades involved in classic and mannan-binding ligand complement activation, contact activation and intrinsic coagulation. C l-Inh inhibits diverse proteases: Clr and Cls, MASP-1, MASP-2, kallikrein, FXIIa, FXIa, and plasmin. Deficiencies in Cl-Inh cause profound clinical syndromes. The best known is hereditary angioedema (HAE), a genetic deficiency in Cl-Inh, which is characterized by transient, recurrent attacks of intestinal cramps, vomiting, diarrhea and life-threatening episodes of tracheal swelling. Fluorescenated StcE binds to cultured macrophages, B- and T-cells. Thus, StcE is an example of a growing class of ZMPs such as tetanus, botulinum and anthrax lethal factor toxins. These ZMPs, in contrast to the homologous Pseudomonas and Vibrio ZMPs, have specific, non-extracellular matrix protein targets. We will test the hypothesis that StcE degrades soluble or cell-associated Cl-Inh, and this results in poorly regulated serine protease cascades involving complement activation, contact activation and coagulation. This dysregulation would then contribute to local inflammation, tissue damage and edema. The elucidation of StcE structure and function(s) may result in new targets for chemotherapeutic or immune prevention or treatment of EHEC infections, which now are best managed only by supportive therapy.